1. Field of the Invention
Measuring the activity in and the efficiency of the disc information storage units of a computer
2. Description of the Prior Art
The performance of a computer is important because of its high fixed cost, its high operational cost and its potential for processing information at high speed. Poor performance, in terms of utilization, will not only result in higher operational costs through less work per unit time but also in higher fixed cost through additional equipment needed to handle the work.
Since the theoretical maximum processing capacity of a computer is difficult to determine because of the number of factors affecting it, an assumed maximum capacity is used as an indicator to determine performance. However, even on this basis, determining performance is not easy.
One way is to determine the amount of time, and therefor the percentage of time, that various electrical circuits within the computer are operating. This is done by two principal methods.
The first is to connect wires to specific probe points within these circuits. The wires are connected to external recording devices which will then show when current, and therefore information, is passing through the circuit. This is usually done by measuring the voltage drop in the circuit.
For example, an electrical circuit that would be monitored is the circuit between information storage and either the computer processing section or an external source. The greater percentage of time that information is passing through the circuit, as indicated by the current within the circuit, the better the performance of the computer.
In most large computers this information, both processable information and processing or program information, is stored on discs in an array of cylinders. The cylinders are best represented by concentric rings on the surfaces of the discs. There may be as many as five hundred cylinders of information on a disc and five thousand or more bits of information on each cylinder. Usually, a number of discs, from two to twenty, are mounted together as a unit and rotate on a single axis. There may be as many as one thousand of these units in a computing facility. Three to four hundred units are common.
For each side of a disc there is a corresponding read-write head. The head reads or carries information stored on a cylinder to the computer information processing section or to an external source, and writes or carries information either from an external source or the computer information processing section to a cylinder for storage.
Each head is mounted on an arm. The arms for a unit of discs are integral and form a comb. The comb moves all the heads together radially back and forth among the cylinders on the discs. Only information to or from a single cylinder on one side of a disc in a unit of discs may be transferred, or accessed, at any one time.
One method of determining the performance of the disc storage, and the computer, is to attach wires to the electrical circuits of the heads and determine the amount of time that current, or information, is passing through the heads.
There are a number of problems that can occur when wires are attached directly to these circuits, or directly to any electrical circuit within the computer.
The major problem is the information passing through the circuit may be changed when external wires are attached. Extraneous current from the wire may pass into the circuit and be interpreted as information by the computer information processing section or the storage system. If there is no information passing through the circuit, the extraneous current may supply information when there should be none. If there is current, or information, passing through the circuit, the extraneous current may add or subtract from the current in the circuit and change the information passing through the circuit. These changes will result in reduced performance because the information must be reprocessed. Often the computer will stop because the information or program is unprocessable.
It is also difficult to place the wires properly. Several man days are required because of the number of wires. The probe points must be located and tests made to determine whether they have been located correctly.
The possibilities for error are great when testing for performance this way.
A second method is to make a continuous record of the transactions occurring in the system. A software program is introduced into the system. The program requires that any transaction be recorded on a separate tape or disc file. The location of the transaction will also be recorded. The file is later analyzed to determine the performance of the computer. A practical problem is that the analysis occurs long after the transactions that were monitored, and the conditions within the computer and information storage system at the time of analysis may not be the same as at the time of monitoring. It is an attempt to improve the performance of a filing system after many of the old files, on which the performance evaluation had been based, have been replaced with new files.